1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to radio frequency integrated circuits used within such wireless communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
Many of the components in the transmitter and receiver are adjustable to compensate for integrated circuit process variations, transmit power requirements, and/or varying levels of received signal strength. For example, the low noise amplifier and filtering stages of a receiver are adjustable to accommodate for varying levels of received signal strength. As is known, to adjust the gain of a low noise amplifier and/or filter stage, the signal strength of the received signal needs to be measured.
Typically, the signal strength is measured via a received signal strength indication module, which measures the amplitude (in volts) of the in-phase and quadrature components a received signal and converts the measured voltage into a power level value (in dBm, i.e., milliwatts of power in dB). For example, the received signal strength indication module uses the equation 10 log (I2)+10 log (Q2) to approximate the equation of 10 log (I2+Q2), where I corresponds to the amplitude of the in-phase component of the received signal and Q corresponds to the quadrature phase of the received signal. This approximation provides a reliable RSSI value since the power of I is equal to the power of Q and does not change over time.
In direct conversion receivers (i.e., those that include only one mixing stage and directly convert received RF signals to baseband signals), timely and accurate measure of RSSI is difficult. The difficulty arises when a frequency offset (i.e., the local oscillation of the receiver does not exactly match the local oscillation of the transmitter of the transmitting radio) is present, causing an amplitude modulation of the received signal, which causes the power of I to not always equal the power of Q. As such, the approximation as previously discussed is not directly viable for direct conversion receivers.
One solution for direct conversion receivers is to average the RSSI of a plurality of samples over a period of time greater than the period of the amplitude modulation caused by the frequency offset. However, when the frequency offset is relatively short (e.g., less than a few hundred kilohertz), the time to obtain a reliable average for the RSSI value exceeds the time allocated to make an RSSI measurement, which typically is in the range of about 1–5 microseconds.
Therefore, a need exists for a method and apparatus for determining received signal strength indication within a direct conversion receiver in a timely and efficient manner.